We define that two objects are similar if they satisfy
a two-place conceptual equivalence predicate (defined below), which is
independent of the Lisp image so that the two objects in
different Lisp images can be understood to be equivalent under
this predicate. Further, by inspecting the definition of this conceptual
predicate, the programmer can anticipate what aspects of an object
are reliably preserved by file compilation.

The set of objects that are externalizable objects are those
for which the new conceptual term "similar" is defined, such that
when a compiled file is loaded, an object can be constructed
which can be shown to be similar to the original object which
existed at the time the file compiler was operating.